1. Field of the Invention
The present invention relates to a scheme for controlling light-emitting diodes (LED), and more particularly, to a controlling apparatus and related lighting module for controlling a plurality of LED strings.
2. Description of the Prior Art
Recently, light sources implemented with light-emitting diodes (LED) have become more popular. For example, light sources in a backlight module in a conventional liquid crystal display (LCD) panel are usually implemented with cold cathode fluorescent lamps (CCFL). However, as the optical efficiency of an LED increases repeatedly, and cost of the LED decreases continuously, the cold cathode fluorescent lamps are replaced by light-emitting diodes gradually as light sources in a backlight module due to their being more economical.
In the prior art schemes, multiple light-emitting diodes are connected in sequence for reducing the number of required driving circuits, and for decreasing a total driving current utilized for driving the light-emitting diodes. However, because of fabrication process variations of different light-emitting diodes, it is very difficult to ensure that parameters of the light-emitting diodes in different LED strings are identical. Additionally, the parameters of the light-emitting diodes may usually be affected by some environmental factors (e.g. temperature). For instance, the forward voltages (VF) of different light-emitting diodes are usually a little different due to the above-mentioned factors. Therefore, the scheme utilizing multiple light-emitting diodes connected in sequence to be an LED string will accumulate forward voltage differences caused by different light-emitting diodes in the LED string. Usually, the total voltage differences accumulated by the forward voltage differences in different LED strings are also different.
In this situation, even though an identical operating voltage is applied for driving all LED strings, currents passing through different LED strings are also different since the accumulated values of the forward voltage differences in different LED strings are not identical. Therefore, the brightness of different LED strings will be different because currents passing through different LED strings are not identical. If the above-mentioned LED strings are used as light sources in a backlight module in an LCD panel, inconsistencies will be introduced on the display area of the LCD panel, since the brightness of the lighting source in the backlight module is uneven.